Driver circuit for a D.C. motor without commutator

ABSTRACT

A method is disclosed for the low-loss regulation of a d.c. motor without commutator and of a semiconductor circuit in which, during a commutation phase at reduced motor power or rpm as given by a position indicator, the end transistors or one end transistor initially operates temporarily as a switch and thereafter temporarily as an analog amplifier element. During the analog period, a current is available which changes slowly according to a ramp function.

BACKGROUND OF THE INVENTION

The invention relates to a driver circuit for the electronic control ofthe rpm of brushless d.c. motors by influencing the motor power by wayof the ratio of the ON duration to the OFF duration of current pulsesfed to at least one motor winding, with the ON and OFF edges of thevariable duration current pulses being controlled in a rampshapedmanner.

Such a driver circuit is disclosed, in particular, in FIG. 7 of WO87/02528.

SUMMARY OF THE INVENTION

Based on this state of the art, it is the object of the invention toprovide a driver circuit which permits a constant rpm without thedevelopment of noise, independently of the load and motor tolerances andindependently of the motor parameters and which additionally is composedof the least number of easily integrated components.

This is accomplished according to the invention in that signalproportional in frequency to measured rpm is used as a measure of therpm for rpm regulation; the rpm proportional frequency is fed to afrequency/voltage converter which generates a signal value that changesmonotonously within one period duration and whose limit value at the endof the period duration can be fed to a comparator stage which influencesthe ON-durations of the current pulses fed to the stator windings of themotor. A triangular wave generator is provided to control theramp-shaped current curve by generating a triangular wave signal whosepeak lies at least approximately in the middle of a one period durationdefined by the frequency/voltage converter, with the triangular-wavegenerator receiving its informations for forming the peak from themonotonously changing signal of the frequency/voltage converter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference toan embodiment that is illustrated in the drawing figures. It is shownin:

FIG. 1, a block circuit diagram of the driver circuit according to theinvention;

FIG. 2, the sawtooth generator of the driver circuit according to FIG.1;

FIG. 3, the triangular-wave generator and adjacent components of thedriver circuit according to the invention;

FIGS. 4a-4h an overview of the voltage/current curves during a timeperiod at or between various points in the circuit of FIGS. 1 to 3 toexplain the operation, including

FIG. 4a which shows the time curve of the output signal of Hall element14;

FIG. 4b and 4c which show the Hall element pulse from Hall signalamplifiers 15 and 16;

FIG. 4d which shows the sum Hall signal pulse behind coupling resistors29 and 30;

FIG. 4e which shows the sawtooth voltage U₂₃ at the output of generator19;

FIG. 4f which shows the "charging voltage" at the charging capacitor 45without a superposed signal from triangular-wave generator 32;

FIG. 4g which shows the output signal U₄₆ of delta generator 32; and

FIG. 4h which shows the actual voltage course at point 48;

FIG. 5a which shows the current curve during phase 1 of the motor; and

FIG. 5b which shows the current curve during phase 2 of the motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The block circuit diagram shown in FIG. 1 shows a driver circuit for atwo-phase, two-conductor, brushless d.c. motor, including a first statorwinding 1 and a second stator winding 2. The d.c. motor has, forexample, a power range up to about 3 watts and a rated input voltage of12 volts. The ends of stator windings 1, 2 are connected via a diode 3with a terminal 4 for the positive pole of the operating voltage source.

As can be seen in FIG. 1, stator windings 1 and 2 respectively lie inthe collector circuit of a first power transistor 5 and the collectorcircuit of a second power transistor 6. The emitters of powertransistors 5 and 6 are connected with one another and, via an emitterresistor 7 acting as a safety resistor and as a feedback resistor forthe analog current control to be described below, to a terminal 8 forthe negative pole of the operating voltage source. By alternatinglyperiodically actuating power transistors 5 and 6, magnetic fields arealternatingly generated by stator windings 1 and 2 to cause thepermanently magnetic rotor of the brushless d.c. motor to rotate. Theturn-off voltages are here limited by Zener diodes 9 and 10 which bridgethe base-collector paths of power transistors 5 and 6.

The base of the first power transistor 5 is connected with the emitterof a first driver transistor 11 serving as analog switch and the base ofthe second power transistor 6 is connected with the emitter of a seconddriver transistor 12 which also serves as analog switch. The collectorsof driver transistors 11 and 12 are connected with the output of aninverter amplifier 13 which furnishes delta (triangular-wave) pulses atperiodic intervals to permit changing the torque of the d.c. motor bychanging the ratio of the duration of the ON-state to the duration ofthe OFF-state within each commutation period of the d.c. motor. Themotor current is here turned on and off in a "smooth" mannercorresponding to the sloped edges of the triangular-wave pulses so as tosuppress as much as possible any switching noises and high frequencyinterferences. During the turning on and off, power transistors 5 and 6temporarily operate as linear amplifiers and, depending on the amplitudeof the delta signal furnished by inverter amplifier 13, for a longer orshorter period of time between the intervals for turning on and off,they operate as switching transistors so that current pulses flowthrough stator windings 1 and 2. These current pulses have approximatelythe shape of trapezoids, with the widths of the trapezoid varying and,in the extreme case, the trapezoids become so narrow that they become atriangle. By alternatingly actuating driver transistors 11 and 12 whichserve as analog switches, it is accomplished that the pulses, which riseand fall according to a ramp function, are fed alternatingly to thefirst stator winding 1 and to the second stator winding 2.

The momentary rotary position of the rotor of the twophase,two-conductor, brushless d.c. motor (not shown in the drawing) isdetected with the aid of a sensor circuit which includes a Hall element14 whose outputs are connected with a first Hall signal amplifier 15 andwith a second Hall signal amplifier 16. Hall signal amplifiers 15 and 16are configured as a double comparator or a double operational amplifierand generate, from the Hall signals, two rectangular signals offset by180° el which, due to the connection of resistors at the inputs of Hallsignal amplifiers 15 and 16, have a slightly asymmetrical shape so thateach pulse signal is somewhat shorter than the pulse pauses and, forexample, a positive pulse signal will never be present simultaneously atthe outputs of Hall signal amplifiers 15 and 16. Thus, a descending edgeof a pulse furnished by one of Hall signal amplifiers 15 or 16 isfollowed, with a short delay by a rising edge at the output of therespectively other Hall signal amplifier 16 or 15.

As can be seen in FIG. 1, the output of the first Hall signal amplifier15 is connected with the base of the first driver transistor 11 and theoutput of the second Hall signal amplifier 16 is connected with the baseof the second driver transistor 12. The alternating occurrence of therectangular Hall signal pulses thus causes the analog switches formed bydriver transistors 11 and 12 to be alternatingly switched on and off sothat alternatingly a current pulse flows through the first statorwinding 1 and the first power transistor 5, on the one hand, and throughthe second stator winding 2 and the second power transistor 6 on theother hand. The torque associated with the current pulses here dependson the time during which the Hall signal pulses and the delta pulsesappearing at the output of inverter amplifier 13 overlap. The deltapulses here have a duration which is never longer than the duration ofthe Hall signal pulses. If the delta pulses have their maximum length, amaximum torque is realized and current pulses having a trapezoidal shapeflow through each one of power transistors 5 and 6 as long as the rpmregulator operates in the intended operating range.

Via a thermal protection circuit 17 and a biasing resistor 18 Hallelement 14 is connected with the operating voltage. Thermal protectioncircuit 17 constitutes an overload protection or a monitoring circuitfor the temperature of power transistors 5 and 6. If the maximumpermissible barrier layer temperature of power transistors 5 and 6 isexceeded, thermal protection circuit 17 enables the driver circuit to beturned off and to be kept in the OFF-state for a certain period of timedue to hysteresis. This is accomplished, for example, simply byinterrupting the current supply to Hall element 14 so that no furtherHall signal pulses are generated and the analog switches formed bydriver transistors 11 and 12 remain in the OFF-state.

The Hall signal pulses furnished by Hall signal amplifiers 15 and 16 notonly serve to control driver transistors 11 and 12 but also assynchronization signals for a sawtooth generator 19 which is shown insimplified form in FIG. 1 and in detail in FIG. 2. Sawtooth generator 19utilizes the frequency of the output signal of Hall element 14 whichserves as a measure for the rpm of the d.c. motor and generates sawtoothpulses whose maximum amplitude changes with the frequency of the Hallsignal pulses and thus with the rpm of the d.c. motor. Additionally, themaximum amplitude of the sawteeth generated by sawtooth generator 19depends on a slope control signal which is fed to the control signalinput 20 of sawtooth generator 19 by a controllable current source 21whose significance will be described below.

Sawtooth generator 19 includes a charging capacitor 22 which is chargedby the current fed in through control signal input 20. The rate at whichcharging capacitor 22 is charged depends on the magnitude of the currentfurnished by controlled current source 21. With increasing charge, thecharging voltage of charging capacitor 22 increases and thus also themomentary voltage of the sawtooth signal present at output 23 ofsawtooth generator 19. FIG. 2 shows that charging capacitor 22 isconnected, on the one hand, with signal control input 20 and, on theother hand, with the base of an impedance converter transistor 24 whosecollector is connected directly, as is charging capacitor 22, with oneof the poles of the operating voltage source. The emitter circuit ofimpedance converting transistor 24 includes an emitter resistor 25 atwhich the output voltage of sawtooth generator 19 is picked up.

Charging capacitor 22 additionally is connected in parallel with thecollector-emitter circuit of a discharging transistor 26 whose base isconnected with the collector of an actuating transistor 27 whose emitteris connected directly with the emitter of discharging transistor 26 andwhose collector is connected via a collector resistor 28 with theoperating voltage source. The base of actuating transistor 27 isconnected via a coupling resistor 29 with the output of the first Hallsignal amplifier 15 and via a coupling resistor 30 with the output ofthe second Hall signal amplifier 16. For that reason, actuatingtransistor 27 switches through whenever pulse pauses exist for bothsignal furnished by Hall signals amplifiers 15 and 16. This is alwaysthe case after one-half an electrical revolution of the d.c. motorfollowing the trailing edge of the Hall signal pulse at the output ofone of Hall signal amplifiers 15 or 16 until the respective other Hallsignal amplifier 16 or 15 furnishes a leading edge. During this time,charging capacitor 22 is kept discharged. Thereafter the chargingvoltage is able to increase until pulse pauses again reach the base ofactuating transistor 27 via the two coupling resistors 29 and 30.

The above description of sawtooth generator 19 shows that the durationof the sawtooth pulses corresponds essentially to the duration of theHall signal pulses and that the maximum amplitude is greater the slowerthe d.c. motor rotates and the more current is furnished by controlledcurrent source 21. Thus, charging capacitor 22 is charged by thecontrollable or controlled current source 21 and is quickly dischargedduring each commutation process. A sum signal formed from the outputsignals of Hall signal amplifiers 15 and 16 serves to effect thedischarge.

The output 23 of sawtooth generator 19 is connected with the controlinput 31 of a triangular-wave generator 32, with the inverting input 33of a comparator 34 and with the inverting input 35 of an alarmcomparator 36.

The non-inverting input 37 of comparator 34 is connected with a firstvoltage divider including a first resistor 38, a second resistor 39 anda third resistor 40 in such a manner that the non-inverting input 37receives about 75 % of the voltage present across the series connectionof the first resistor 38 and the second resistor 39. These resistors 38and 39 bridge a Zener diode 41 which furnishes a constant referencevoltage as the rpm reference signal even if the operating voltage forthe described driver circuit changes.

The output 42 of comparator 34 is connected via a resistor 43 of, forexample 1 megohm, with the positive pole of the operating voltagesource. Additionally, output 42 is connected via a resistor 44 having asignificantly lower resistance value of, for example, 51 kiloohms with acharging capacitor 45. Resistor 44 and charging capacitor 45 form an RCmember which is charged slowly via collector resistor 43 and resistor 44and with the output 42 of comparator 34 open and is dischargedrelatively quickly via resistor 44 and with the output 42 of comparator34 closed. This charging and discharging occurs at periodic intervalscorresponding to the periodic intervals with which the sawtooth signalat inverting input 33 exceeds the reference voltage at non-invertinginput 37. The average charge or voltage developing at charging capacitor45 is thus a function of the rpm of the d.c. motor. Each time thesawtooth voltage present at inverting input 3 is higher than the rpmreference voltage at non-inverting input 37, charging capacitor 45 isdischarged somewhat, with an average, not quite smooth direct voltagedeveloping at charging capacitor 45 which voltage drops when thedischarge periods become longer due to the rpm going down and thus thesawtooth amplitude becoming higher.

The above described arrangement thus constitutes a frequency/voltageconverter whose output voltage is higher the higher the rpm of the d.c.motor. Fluctuations in voltage within one period here usually lie onlyin a range from 1 to 5% of the average direct voltage and are thusessentially negligible.

As can be seen in FIG. 1, the terminal of charging capacitor 45 notconnected with resistor 44 is not connected to ground but to the output46 of triangular-wave generator 32 which is synchronized with thesawtooth signal via control input 31. For this reason, the chargingvoltage picked up at the connection 48 of resistor 44 with chargingcapacitor 45 has superposed on it the triangular-wave signal voltagefurnished by delta generator 32 with a period duration which correspondsto the period duration of the sawtooth signal. The voltage composed ofthe direct voltage component and the triangular-wave signal voltagecomponent is fed as a control voltage via a resistor 49 to the invertinginput of inverter amplifier 13 which is stabilized by means of aninternal capacitor (not shown in the drawing) and whose gain is definedby the resistor 49 and a resistor 50. For this reason, the ratio ofthese resistors to one another determines the slope of the current riseor current drop if the sum voltage present at connection 48 drops belowthe comparison voltage at the non-inverting input 51 of inverteramplifier 13, with this comparison voltage being fixed by the ratiobetween resistors 52 and 53. If thus the direct voltage component acrosscharging capacitor 45 drops due to the rpm becoming smaller as a resultof, for example, an increase in load, the amplitude of thetriangular-wave signal shifted by the direct voltage component liesbelow the voltage present across non-inverting input 51 for a now longerperiod of time. As a consequence, the width of the delta pulsesappearing at output 54 of inverter amplifier 13 becomes greater so thatthe pulse pause ratio at the base of the respective power transistor 5or 6 selected by the Hall signal generator pulses increases andaccordingly the deltashaped or trapezoidal current pulses through statorwindings 1 and 2 become broader. Due to the now broader current pulsesin stator windings 1 and 2, a higher torque results for the d.c. motorwhich counteracts, for example, the drop in rpm caused by an increase inload. If the rpm rises again, the sawtooth signal pulses become shorterand so do the time periods during which the sawtooth signal at theinverted input 33 of comparator 44 lies above the rpm reference voltageat non-inverting input 37. For this reason, the discharging times ofcharging capacitor 45 become shorter so that the direct voltagecomponent at connection 48 increases again and the pulse durations atoutput 54 become shorter again as the rated rpm is approached.Correspondingly reversed are the relationships if the rpm of the d.c.motor rises above the rated rpm.

FIG. 3 shows in greater detail the part of the circuit includingtriangular-wave generator 32, comparator 34 and alarm comparator 36shown in the block circuit diagram of FIG. 1. Components coinciding withcomponents shown in FIG. 1 bear the same reference numerals.Additionally, FIG. 3 shows an output transistor 55 whose emitter isconnected with the resistor 49. An emitter resistor 56 lies in theemitter circuit.

FIG. 3 also shows how the rpm reference voltage at the tap betweenresistors 38 and 39 is conducted, via an impedance converter transistor57 including an emitter resistor 58, to the non-inverting input 37 ofcomparator 34.

As can be seen in FIG. 1, triangular-wave generator 32 is also chargedwith the rpm reference signal via a reference voltage line 59. Thesawtooth signal is inverted in triangular-wave generator 32 and theinverted signal is fed together with the original signal to an analogcomparison stage which forwards the respective larger one of the twosignals as output signal to output 46. The rpm reference voltage hereserves as a reference for the inversion.

FIG. 3 shows how the functions of triangular-wave generator 32 can berealized with few components, namely two transistors 60 and 61 and threeresistors 62, 63 and 64. Resistors 62 and 63 have approximately the sameresistance values, with resistor 62, for example, having a value of 43kiloohms and resistor 63 a value of 33 kiloohms.

If the sawtooth signal voltage fed from output 23 to control input 31via line 65 lies near 0 volt, transistor 60 is not conductive so that ahigh voltage corresponding to the rpm reference voltage is present atits collector. With increasing sawtooth voltage, transistor 60 becomesmore and more conductive so that its collector-emitter voltage drops andthe voltage drop across resistor 62 increases. The base voltage fed tothe base of transistor 61 drops correspondingly so that the voltageacross resistor 64 drops. During the rise of the sawtooth signalvoltage, a point in time is reached at which transistor 60 has switchedthrough completely and the collector-emitter voltage is very low. Atthis time, the voltage at the collector of transistor 60 has reached itslowest value. When the sawtooth signal voltage increases further, thevoltage at the collector of transistor 60 increases again causing thevoltage at resistor 64 to increase as well. In this way, components 60to 64 generate a synchronized triangular-wave signal. It should be notedthat, due to the above-described regulating circuit, the triangular-wavesignal voltage has a maximum amplitude which lies slightly below thevoltage of the rpm reference voltage which is, for example, 4 volts, sothat an amplitude which triangularly fluctuates between 2 and 4 voltsresults for the triangular-wave signal at the collector of transistor60. The triangular-wave signal is forwarded by transistor 61 whichoperates as an impedance converter and is picked up at resistor 64 sothat the charging voltage signal can be superposed on it in chargingcapacitor 45.

FIG. 3 also shows how alarm comparator 36 is connected with theoperating voltage via a light-emitting diode 67 and a resistor 66.

As mentioned above, current source 21 is a controllable or controlledcurrent source. By changing the current furnished by current source 21,it is possible for the circuit shown in FIG. 1 to specify atemperature-rpm characteristic corresponding to a temperature detectedby an NTC resistor 70. To accomplish this, in addition to NTC resistor70, resistors 71, 72, 73 and 74 are provided which furnish, viasymbolically shown linkage circuits or comparison circuits 75 and 76 andvia a control line 77, a control signal to controllable current source21 in dependence on the ambient temperature. The arrangement shown inFIG. 1 makes it possible, for example, to obtain a constant rpm for thed.c. motor at temperatures up to 30°, with such rpm being independent ofthe motor parameters and independent of the load of the motor. Only ifthe temperature detected by NTC resistor 70 increases beyond 30°, iscurrent source 21 adjusted so that with increasing temperature anincreasing rpm is realized. The maximum rpm is reached, for example, at50°. Thereafter, the rpm remains constant again, preferably somewhatbelow the maximum rpm the motor is able to attain without regulation.

The analog comparator 75 which compares the voltage drop across NTCresistor 70 with that across the voltage divider composed of resistors72, 73, 74, forwards the smaller one of these voltages as its outputsignal. This smaller voltage is associated with an rpm below, forexample, 30°. If the temperature increases to above 30° C., the voltagedropped across NTC resistor 70 is the lower voltage and is forwarded bycomparator 75 to comparator 76. If the voltage which is determined bythe temperature value at NTC resistor 70 drops below a value associated,for example, with a temperature of 50°, comparator 76 forwards thevoltage picked up between resistors 72 and 73 which is constant anddetermines the maximum rpm.

FIG. 4a shows a voltage curve U_(141/142) at the outputs 141 and 142 ofHall element 14. Hall signal amplifiers 15 and 16 generate, from theHall signals, i.e. from the voltage curve U_(141/142), two rectangularsignals U₁₅₀ and U₁₆₀, respectively, which are offset by 180° el, asshown in FIGS. 4b and 4c. A descending edge 158 (trailing edge) isfollowed by an ascending edge 157 (leading edge) at the output of therespective other amplifier 16 or 15.

In each one of the thus formed pulse pauses 155 (FIG. 4d) of the signalsfurnished by Hall signal amplifiers 15 and 16, transistor 27 switchesthrough. After a trailing edge 158 of the Hall signal pulse at theoutput of one of amplifiers 15 or 16, the respective other amplifier 16or 15 furnishes a leading edge 157. During this time, charging capacitor22 is kept discharged. Thereafter, charging voltage U₂₃ (FIG. 4e) isable to rise until the next pulse pause 155.

The curve of sum signal U_(30*) shown in FIG. 4d formed from the outputsignals of amplifiers 15 and 16 acts behind coupling resistors 29 and 30on the base of transistor 27.

If connected to 0 potential (numeral 8) and with the collector ofcomparator 34 open, capacitor 45 would be charged slowly (time constantτ_(charge) ≈20 * τ_(discharge)) and would be discharged quickly (timeconstant τ_(discharge) relatively small) if the collector is closer.This charging and discharging takes place periodically in the samemanner as sawtooth signal U₃₃ (=U₂₃) at the inverting input 33 exceedsthe reference voltage U₃₇ at the non-inverting input 37.

However, charging capacitor 45 is connected with the output 46 of deltagenerator 32. Therefore the "charging voltage" acting on point ofconnection 48 has superposed on it the delta signal voltage U₄₆furnished by delta generator 32 (shown in FIG. 4g). The voltage U₄₈combined of the charging voltage component U_(48*) and the delta signalvoltage component U₄₆ acts via resistor 49 on the input of inverteramplifier 13.

FIG. 4f shows the "charging voltage" component U_(48*) and its averageU_(48*).

FIG. 4h shows the actual curve of voltage U₄₈ at point 48. It is formedof the alternating component of U₄₆ (see FIG. 4g) and the direct voltagecomponent at connection 48 (shown in FIG. 4f as U_(48*) together withits average U_(48*)).

Preferably, current source 21 is a current source which is controllableor controlled by an NTC resistor 70. By changing the current furnishedby current source 21, the amplitude of sawtooth signal 23 is changed andit is possible with the circuit shown in FIG. 1 to predetermine atemperature/rpm characteristic depending on the temperature detected byan NTC resistor 70.

FIGS. 5a and 5b show the typical current curve during phase 1 and 2,respectively, of the motor.

I claim:
 1. A device for electronically regulating the speed of a rotorof a brushless d.c. motor having at least one stator winding, the motorhaving an on state in which a current in the form of a pulse havingramped on and off edges is fed to the at least one stator winding and anoff state in which a current is not fed to the stator winding, thedevice comprising:a frequency to voltage converter having means,responsive to a frequency-modulated signal whose frequency isproportional to the speed of the motor, for generating a periodic signalhaving a signal value which changes monotonically during each period ofthe periodic signal and has a limit value at the end of the period ofthe periodic signal which is proportional to the speed of the motor;means for applying current pulses to the stator winding at a rate whichis proportional to the speed of the motor; a comparator stage havingmeans, responsive to the periodic signal, for generating a comparatoroutput signal having a value which is proportional to the limit value atthe end of each period; and a triangular-wave generator having means,responsive to the periodic signal during each respective period, forgenerating a triangularly shaped signal having a peak formed atapproximately the midpoint of the duration of the period, said applyingmeans having means, responsive to the comparator output signal and saidtriangularly shaped signal, for controlling the duration of each of thecurrent pulses, the duration controlling means including means,responsive to the triangularly shaped signal, for controlling the on andoff edges of each current pulse.
 2. A device as in claim 1, wherein saidcomparator stage includes:means for producing a d.c. component which isproportional to the limit value, and superposing the triangularly shapedsignal and the d.c. component to form a superposed signal, and means,responsive to the superposed signal and the frequency-modulated signal,for forming the current pulses at the frequency of the frequencymodulated signal and with durations which are controlled by thesuperposed signal.
 3. A device as in claim 1, wherein the at least onestator winding includes two stator windings and the current pulses areapplied to the respective two winding at a rate which is invariablycoupled to the value of the speed of the motor of 1:2.
 4. A device forelectronically regulating the speed of a permanently magnetic rotor of abrushless d.c. motor having at least two poles, at least one statorwinding, and a sensor which outputs a periodic sensor signal indicativeof rotor position, the device comprising:an end stage having means fortemporarily operating as a switch and temporarily operating in ananalogue domain; a sawtooth-wave-generator having means, responsive tothe periodic sensor signal, for generating a periodic sawtooth-wavevoltage of predetermined slope; a comparator having an output port andhaving means, responsive to the sawtooth-wave voltage, for comparing thesawtooth-wave voltage with a motor speed reference signal voltage andoutputting a comparison signal indicative of which of the sawtooth-wavevoltage and motor speed reference signal voltage is higher, thecomparison signal having a d.c. component; a summation device; alow-pass filter connected to the output port of said comparator fortransferring therethrough the d.c. component; a triangular-wavegenerator having means, responsive to the periodic sawtooth-wavevoltage, for generating a triangularly shaped signal synchronized withthe sawtooth-wave voltage; means for coupling the d.c. componenttransferred through said filter and the triangularly shaped signal tosaid summation device, said summation device having an output signalwhich represents a sum of the d.c. component and the triangularly shapedsignal; and a linkage circuit having means, responsive to the outputsignal of said summation device and the sensor signal, for generatingduring each period an end stage control signal which is variable in timeand has a duration which is shorter than the duration of the sensorsignal; said end stage comprising means, responsive to said end stagecontrol signal, for generating current pulses of duration shorter thanthe duration of the sensor signal, with ramp-shaped leading andfollowing edges, for application to the at least one stator winding. 5.A device as in claim 4, wherein said summation device comprises at leastone operational amplifier.
 6. A device as in claim 4, wherein thelowpass filter comprises an RC lowpass filter.
 7. A device as in claim6, wherein said RC lowpass filter includes a capacitor, first and secondresistors, the first resistor having a greater resistance than thesecond resistor, means for charging said capacitor through said firstresistor, and means for discharging said capacitor through said secondresistor.
 8. A device as in claim 7, wherein said capacitor coupled anoutput port of said triangular-wave generator to an input port of saidsummation device.